**2. Epidemiology of** *Campylobacter* **infections**

#### **2.1 Incidence and distribution**

Campylobacteriosis has been reported to be prevalent in both domestic and wild birds but the former is considered to be more affected [5]. The possible reason might be that transmission among domestic birds, especially commercial poultry, is high due to more number of birds in a unit area. Different factors in commercial farming may affect the occurrence of campylobacteriosis, including the type of farming, housing system, region, and biosecurity measures. It has also been reported that the prevalence of campylobacteriosis is high in months in which the temperature is high, resulting in a higher population of flies and higher flies-mediated transmission [6]. Developing countries and European countries are considered to have a high prevalence of the disease as compared to Scandinavian countries [7]. The age of the birds, irrespective of the species and production system, is related to the occurrence of the disease and it being less likely to occur in birds of less than 2–3 weeks of age. The occurrence of *C. jejuni* isolates among other *Campylobacter* species is high followed by *C. coli* and *C. lari* [8]. The isolation of other species of *Campylobacter,* including *C. upsaliensis* and *C. hyointestinalis*, is very low from poultry [1, 5].

#### **2.2 Transmission, carriers, and vectors**

Horizontal transmission is the most common mode by which transmission of campylobacteriosis takes place. Vertical transmission does not occur or occurs very rarely. The possible sources for horizontal transmission from the environment to poultry include contaminated water [9], litter especially old litter [10], farm workers [11], contaminated footwear, insects [12], wild animals [13] especially rodents [14], farm animals [15], scavenger birds [16], feed contaminated with feces of chicks [15], house flies [17], visitors, and various types of equipment.

*Campylobacter* is usually excreted through feces, which may contaminate the feed and litter. Survival of *Campylobacter* in a litter depends upon temperature, moisture, and pH [18, 19], although *Campylobacter* can survive in the litter for a minimum of 10 days at 20°C [20]. An infected water supply may also result in the spreading of the disease among the flocks. Insects such as houseflies, darkling beetles, cockroaches, and mealworms play an important role as a mechanical vector in the transmission of *Campylobacters* [21, 22].
